Vapor permeation is a membrane-based process that can be used to separate mixtures of vapors. In an example of such a process, a vaporous mixture of Vapor A and Vapor B is fed to the feed side of a membrane, while a vacuum pump or gaseous sweep stream, usually in combination with a condenser, maintains a sufficiently low partial pressure of Vapor B on the permeate side of the membrane to provide a chemical potential gradient of Vapor B across the membrane. Principally Vapor B, and some Vapor A, are transported to the permeate side of the membrane to form a vapor-phase permeate.
Key to the development of a low-cost, efficient vapor-permeation process is the method used to maintain a low partial pressure of Vapor B on the permeate side of the membrane. The prior art describes the application of a vacuum to the permeate side of the membrane, reducing the total pressure of the permeate, thereby reducing the partial pressure of Vapor B on the membrane's permeate side. However, in many instances the cost and complexity of a vacuum system makes this impractical. Furthermore, vacuum-driven systems often leak, allowing air to enter the system. For many separations, especially those with oxygen-sensitive compounds or highly flammable compounds, the presence of oxygen is undesirable or dangerous. Thus, alternative methods are desirable.
U.S. Pat. No. 4,978,430 discloses a vapor permeation process for dehydrating and concentrating an aqueous solution containing an organic compound, whereby the permeate is kept under reduced pressure or a "dry inert gas" can be used to reduce the partial pressure.
U.S. Pat. No. 5,226,932 discloses a membrane process for drying noncondensable gases such as air, nitrogen, carbon dioxide or ammonia that uses low vacuum levels and a dry countercurrent sweep gas on the permeate side of the membrane. Commonly-owned U.S. Pat. No. 5,108,464 also discloses a membrane process for drying noncondensable gases such as air, lower hydrocarbons and acid gases using a countercurrent sweep gas, wherein the sweep gas may be introduced to the permeate side of a hollow fiber membrane module at the retentate end, such that it mixes with the permeate as it passes along the membrane and then exits at the feed end of the module.
U.S. Pat. No. 5,034,025 discloses a membrane process for drying water vapor-containing noncondensable gases such as air, carbon dioxide or natural gas that includes maintaining a water vapor partial pressure differential across the membrane, contacting the lower pressure and permeate side of the membrane with a dry organic condensable sweep gas that is immiscible with water, preferably in a countercurrent flow mode, collecting and condensing the sweep gas containing permeated water, thereby forming a two-phase organic-aqueous liquid condensate, then separating the organic and aqueous phases.
As is apparent from the foregoing, the prior art has suggested the use of a countercurrent gaseous sweep stream on the permeate side of the separation membrane. However, no guidelines have been suggested as to what properties this sweep gas should have. It has been discovered that, in order for the use of a counter-current gaseous sweep stream on the permeate side to be practical, it must have a low concentration or a low partial pressure of Vapor B on the permeate side of the membrane. Furthermore, the method of generating gaseous sweep containing the low concentration of Vapor B must be carefully selected so as to maintain a high-performance, efficient, low-cost system.
Many industries, most notably the semiconductor and microelectronics industries, are relying more and more on the use of ultrapure solvents for various drying, cleaning and manufacturing processes. Typically, the yield from these processes is highly dependent on the purity of the solvent used. Contaminants that are common for many of these solvents include water, trace metals, anions and particulates.